I am using Chromecast Audio and MM android app, and playback works really well when streaming local media from my NAS to my stereo (have not compared the quality yet, but will do that), but it stops when the phone rings, some other apps stream audio from media on the phone (videos e.g), plus my phone is using way more than idle power when the app is running in the background (as compared to normal idle). Not really what I was expecting to be honest from casting the media to the Chromecast. I thought the point was to avoid that and not just avoid the wire... :/ will this be addressed in a future update, or has this not been the purpose of integrating casting to the Chromecast? Or is the phone actually streaming (not allocating the casting task to the Chromecast) the music to the chromecast actively all the time, which causes these problems?

Yes, the phone is streaming the music to the Chromecast device. This is not a bug: it can't be any other way since the music being streamed is stored on your phone; so the phone has to be the one to stream it to the Chromecast device.

This differs from, say, Pandora, where the Chromecast device receives the music stream from the Pandora servers directly. So in that case, the phone only has to be involved with controlling the stream (play, pause, volume). But in the case of MMA, the phone has to do it all.

Chromecast doesn't support DLNA and your NAS likely doesn't support Chromecast. As the Chromecast doesn't support DLNA MMA is doing the translation between DLNA from the NAS to whatever Chromecast uses for streaming.

If you had a DLNA client then MMA could function as control point and not do the actual streaming between server and client.

My NAS (a WD My Cloud) has DLNA functionality (a build-in DLNA server), and I have since found out that rendering music files on the Chromecast Audio works for BubbleUPnP android. So I am unsure why it does not seem to work with MMA (if you say it should work Lowlander....).